1. Field of the Invention
The present invention generally relates to the conversion of chemical energy to electrical energy and, more particularly, to an alkali metal electrochemical cell having a cathode pellet formed at a higher pressure than dictated by conventional practice while maintaining an acceptable electrolyte-to-cathode (E/C) weight ratio and anode-to-cathode (A/C) capacity ratio. A preferred cathode includes a solid active material and a most preferred cathode active material is a carbonaceous material, such as fluorinated carbon.
2. Prior Art
A prior art Li/fluorinated carbon cell, particularly a Li/CFx cell constructed according to U.S. Pat. No. 5,716,728 to Smesko et al., which is assigned to the assignee of the present invention and incorporated herein by reference, is typically housed in a conductive casing having nominal dimensions of 45 mmxc3x9723 mmxc3x975 mm (referred to as a 5 mm Li/CFx cell) . This Li/CFx cell is known to have an electrolyte-to-cathode (E/C) weight ratio of about 0.94 to 0.96 and an anode-to-cathode (A/C) capacity ratio of about 1.03. Both the E/C weight ratio and the A/C capacity ratio are based on the gram amount of electrode active material. In other words, the Smesko et al. cell has about 3% excess lithium (in terms of deliverable capacity).
A Li/CFx cell built according to Smesko et al. has a volumetric energy density of about 1,066 Wh/L, a gravimetric energy density of about 519 Wh/kg, an open circuit voltage of 3.0 to 3.3 volts and an estimated self-discharge rate of less than 1% per year. The cell will not rupture or leak when subjected to short circuit conditions at room temperature or at 37xc2x0 C. The volumetric and the gravimetric energy densities are based on the internal volume and total weight of the active components, respectively.
Interestingly, the Smesko et al. cell is itself an improvement on the Li/CFx cell described in U.S. Pat. No. 5,250,373 to Muffoletto et al., which is assigned to the assignee of the present invention and incorporated herein by reference. The Muffoletto et al. cell has a E/C weight ratio of 1.32 based on the gram amount of cathode active material. In the Muffoletto et al. cell, the presence of increased amounts of electrolyte with respect to the gram amount of cathode active material was thought necessary to maintain the internal impedance of the cell low, especially as end-of-life neared. The Smesko et al. cell showed for the first time that excess amounts of electrolyte are not necessary in order to maintain efficient discharge throughout the life of a Li/CFx cell.
In an application where a premium is placed on increased energy density, such as in power sources associated with implantable medical devices, there is a need for an alkali metal/fluorinated carbon electrochemical cell which provides improved volumetric energy density and increased discharge efficiency over those of the prior art, including the Smesko et al. and Muffoletto et al. cells. The cell of the present invention fulfills this need by having a comparable anode-to-cathode capacity ratio but a reduced electrolyte-to-cathode weight ratio that optimizes the electrochemical reactions at the electrodes. This is brought about by pressing the cathode at a higher pressure than is currently practiced in the prior art cell of the Smesko et al. patent.
According to the present invention, the pressed density at which carbon monofluoride cathodes are formed is used to control the shape of the discharge profile and, thereby, enhance the cell""s delivered capacity, especially when such material is used in implantable medical applications. As a majority of implantable medical devices operate above a 2.0 V cutoff, improved delivered capacity enhances device performance and increases device longevity. Increased delivered capacity is realized by using a cathode electrode having the same chemical composition, formulation, and weight as in the state-of-the-art or prior art cells, but pressed at a higher pressure. Increasing the compaction of the cathode mixture increases the electrode""s pressed density. Using the same cathode active material weight, anode active material weight and electrolyte weight as in the Smesko et al. cell, the cell of the present invention delivers about 6% to about 15% additional capacity when the cathode is fabricated having an increased pressed density and the resultant cell is discharged under a 1 kohm load. When discharged under a 2 kohm load, about an additional 2% to about 5% delivered capacity is realized.
Accordingly, increasing the pressed density of the cathode is used to add additional cathode active material to the cell (increase the cathode""s active weight). This results in an increased theoretical volumetric energy density for a cathode-limited cell. The anode/cathode capacity ratio of the cell is also maintained by increasing the amount of anode active material used. This is important in implantable device power sources because the cell voltage gradually falls as end-of-service is approached. Increasing the cathode press density allows the volume available for the anode active material to be expanded or increased, thus allowing the A/C capacity ratio to be maintained without detracting from the end-of service signature of the cell chemistry.
Conventional reasoning would hold that if all of the inactive cell components of a particular cell design are left unchanged, an increased cathode press density would dictate that the electrolyte-to-cathode weight ratio may actually not be able to be maintained because of the additional cathode and anode active materials.
Therefore, conventional reasoning says that the E/C weight ratio would decrease resulting in reduced cell performance under certain high rate applications. However, in the present Li/CFx system, this does not occur. Carbon monofluoride cathodes of a higher pressed density according to the present invention have been found to absorb less electrolyte but are still able to utilize what electrolyte there is efficiently enough to maintain ion mobility during cell discharge.
These and other objects of the present invention will become increasingly more apparent to those skilled in the art by reference to the following description and to the appended drawings.